Science Help

Position Isomerism

Molecules that have the same molecular formula, but have a different arrangement of the atoms in space are known as isomers. It does not includes any different arrangements which are formed due to the molecule rotation as a whole, or rotation about particular bonds.


In structural isomers, the atoms and functional groups are joined together in different ways, as shown in the example of propyl alcohol. It occurs as three isomers:propan-1-ol or n-propyl alcoholpropan-2-ol or isopropyl alcoholmethoxyethane or methyl-ethyl-etherposition isomerismIsomers of propyl alcohol(position isomerism)Position isomerismIn position isomerism, the basic carbon skeleton remains constant, but important groups are moved around on that skeleton. Example: Two structural isomers with the molecular formula C3H7Br are there, bromine atom is on the end of the chain in one of them, whereas in the other it's attached in the middle. 


 position isomerismIf a model is prepared, it is not possible to twist one molecule to turn it into the other one. It would be necessary to break the bromine off the end and re-attach it in the middle. At the same time, hydrogen has to be shifted from the middle to the end.Another example occurs in alcohols such as C4H9OHposition isomerismTwo other isomers of butanol are as follows:position isomerismBenzene rings also show position isomerism. In the molecular formula C7 H7 Cl, there are four different isomers depending on the position of the chlorine atom. 


In first isomer case chlorine is attached to the side-group carbon atom, and then there are three different possible positions it could have around the ring - next to the CH3 group, next-but-one to the CH3 group, or opposite the CH3 group.position isomerismBecause substituent’s can be positioned on different parts of the benzene ring, many aromatic isomers exist. There exists only one isomer of phenol or hydroxybenzene, but cresol or methylphenol exists as three isomers in which the additional methyl group can be placed on three different positions on the ring. Xylenol consists of only one hydroxyl group and two methyl groups and a total of 6 isomers exist.